perf, x86: Extract PEBS/BTS allocation functions

Mostly a cleanup.. it reduces code indentation and makes the code flow
of reserve_ds_buffers() clearer.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Stephane Eranian <eranian@google.com>
LKML-Reference: <20101019134808.253453452@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>